Seeding a pulsed fiber laser system with a directly modulated laser diode is a simple and cost-effective manner of generating high energy, high peak power optical pulses with fast rise times and fall times. In material processing applications where a single pulse is used to process a given structure, such as severing links for memory repair, it is important to keep the energy of the light pulses within a given range. If the pulse energy is too low, then the link may be incompletely removed. In cases where the energy per pulse exceeds the allowable energy process window, excess pulse energy may be coupled into adjacent or underlying link structures, or the substrate itself, thereby causing highly undesirable damage to the device. In multiple pulse laser processes, such as laser drilling of microvias in semiconductors, or laser scribing of thin film photovoltaic devices, it is important that successive pulses remain substantially uniform in order to produce laser processed features that possess the desired dimensions and surface quality. High value is placed upon the throughput of work pieces satisfactorily produced by a laser processing system. The control of the pulse amplitude stability is also important in laser surgery, as the amount of energy deposited in living tissues must be accurately controlled in order to avoid causing damage to the neighbouring tissues. Therefore, methods to achieve pulse stabilization in lasers, particularly in lasers employed in laser processing or medical laser systems, are highly desirable.
Optical pulse shaping is of great interest in material processing applications as it offers the ability to control how the energy is delivered to the target over time. Industrially important laser applications, such as laser repair of dynamic random access memory (DRAM), laser scribing of photovoltaic cells, and laser drilling of microvias in semiconductor, flexible interconnects, IC packages, dielectrics, including glass, and metals, can benefit from a pulsed laser output characterized by tailored pulse shapes. For example, U.S. Pat. No. 7,348,516 (SUN et al.), entitled “Methods of and laser systems for link processing using laser pulses with specially tailored power profiles” presents many arguments in favour of pulsed laser systems providing fine control over the pulse temporal power profile in the nanosecond regime, for facilitating better link process quality and yield. Those skilled in the art will recognize that laser sources capable of generating sharp optical pulses with stable, tailored amplitude profiles characterized by fast rise times and fall times at the nanosecond time scale are highly desirable for such purposes. Elaborate optical pulse shapes require a highly responsive optical shaping mechanism. Pulsed lasers based on a directly modulated laser diode seeding a chain of optical amplifiers in a Master Oscillator, Power Amplifier (MOPA) configuration are commonly employed in these and similar laser processing applications in which laser processing pulses with controllable pulse amplitude profiles are highly beneficial (See for example U.S. Pat. No. 6,281,471 (SMART), entitled “Energy-efficient, laser-based method and system for processing target material”. Embodiments of pulsed laser oscillator platforms employing MOPA configurations with a directly modulated seed laser diode and providing fine control over the pulse parameters are presented in the international patent application published under WO 2009/155712 (DELADURANTAYE et al), entitled “Digital laser pulse shaping module and system” and in international patent application No. PCT/CA2009/00365 (DELADURANTAYE et al), filed on 20 Mar. 2009, entitled “Spectrally tailored pulsed fiber laser oscillator”.
For pulsed laser systems employing a directly modulated seed laser diode, a commonly encountered difficulty is obtaining a satisfactory optical pulse amplitude stability on the leading edge of the pulse, as switching transients often take place in the diode during the transitory regime corresponding to the leading edge of the current pulse. The pulse amplitude stability usually worsens as the pulse rise time is shortened and as the pulse amplitude is increased. Similar transients can occur at the falling edge of the pulse, and generally, transients can be observed for each low to high or high to low transition present in a given pulse shape. Those transients may find their origin in the optical gain switching dynamics taking place in the laser diode cavity as longitudinal mode competition occurs when the drive current is suddenly increased from zero to a value that is above the laser emission current threshold. See for example “Mode switching of Fabry-Perot laser diodes”, by P. J. Herre and U. Barabas, in IEEE Journal of Quantum Electronics, Vol. 25, No. 8, August 1989, pp. 1794-1799. FIG. 1 (PRIOR ART) shows an example of the temporal shape of an outputted optical pulse where such transient behavior is manifested by a spike present at the pulse leading edge. In general, the spike amplitude varies from pulse to pulse, leading to poor peak power stability at the pulse leading edge. Parasitic capacitances and inductances associated with the details of the laser diode physical characteristics and packaging can also contribute to create noise at the current transitions.
In a MOPA configuration (Master Oscillator Power Amplifier), any undesirable features present at the seed level will be amplified. As those skilled in the art will recognize, this effect is often worsened when the pulsed oscillator output is amplified and frequency converted to one or more harmonic wavelengths using the process of nonlinear harmonic conversion, as is well known in the art. Other issues can arise from the presence of peak power instabilities. For example, when fiber amplifiers are used to amplify the pulsed seed signal, excessive peak power induced by the transient behavior can trigger the onset of Stimulated Brillouin Scattering (SBS) or other nonlinear processes in the fibers, which can degrade the performances of the laser and in some cases even cause damage to it. It is consequently of very high interest to have methods for controlling the switching transients of a laser diode operated in the pulsed mode in a very predictable way in the context of flexible pulsed laser oscillators based on a MOPA architecture.
Previous works have described certain methods for mitigating switching transients in other fields of use, such as laser diode drivers used in CD-recorders. For example, CLAVERIE, in European Patent No. 0,053,974, discloses a method to reduce the effects of the switching transients by superposing to the information-containing current pulses a d.c. current with a slightly smaller amplitude than the laser-threshold current. However this method is not very attractive for pulsed lasers having a MOPA architecture and relying on a directly modulated semiconductor laser, because the pedestal (d.c.) current would generate a continuous wave emission background between the pulses, which in turn would deplete the population inversion in the subsequent amplifier stage by stimulated emission. This amplifier gain reduction can substantially reduce the energy delivered in each pulse, especially for regimes of operation corresponding to low duty cycles (e.g. 10% or less), since the relative proportion of energy contained in the pedestal with respect to the energy contained in the pulse increases as the duty cycle is reduced. Such low duty cycle operation regimes are not exceptional for many material processing applications, where pulse durations in the range of 1 to 50 ns at repetition rates of a few hundreds of kilohertz are commonly required by practitioners. Another drawback associated with the continuous background is that the optical power emitted between the triggered optical pulses can cause damage to the work piece between the different targets. As will be recognized by those skilled in the art, the maximum energy impinging on the work piece between targets must be kept lower than a certain threshold value, above which scorching of the material begins to take place.
Even in its field of use, the method of CLAVERIE suffers from the drawback of shortening the lifetime of the semiconductor laser, as mentioned by DUFOUR in U.S. Pat. No. 4,817,097, entitled “Method of and device for pulse-mode driving a semi-conductor laser”. Alleviating the lifetime issue, DUFOUR's approach consists of using two superposed current pulses. A first pedestal current pulse generated by a first current source is used to drive the laser diode into the LED region (that is, using a current lower than the laser-threshold current). A second current source is used to generate an information current pulse that is superposed to the pedestal current pulse, both current pulses being synchronized. This two-stage drive approach allows for a reduction of the switching transients while avoiding unnecessary power dissipation between the information pulses, which is beneficial as it extends the laser diode lifetime. Although this method would certainly work better than the method of CLAVERIE in the context of pulsed laser oscillators having MOPA architectures (as the inter-pulse background would not be present), it imposes a more complex laser diode driving circuit, which represents an additional cost. Furthermore, as those skilled in the art will recognize, an increased component count in the vicinity of the laser diode package usually represents additional difficulties for reaching fast rise times and fall times, because the complex impedance usually increases along with the circuit footprint.
Therefore, improved pulse stabilization methods well-adapted for flexible laser oscillators based on directly modulated seed laser diodes are of substantial interest and value to practitioners in many industrially important applications.